"A security token service (STS) is the service component that builds, signs, and issues security tokens according to the WS-Trust and WS-Federation protocols. There’s a lot of work that goes into implementing these protocols, but WIF does all of this work for you, making it feasible for someone who isn’t an expert in the protocols to get an STS up and running with very little effort. You can use a cloud STS such as a LiveID STS, a pre-built STS such as Active Directory® Federation Services (AD FS) 2.0, or, if you want to issue custom tokens or provide custom authentication or authorization, you can build your own custom STS using WIF. WIF makes it easy to build your own STS." (http://msdn.microsoft.com/en-us/library/ee748484.aspx)